122 research outputs found
Quantum Friction of Micromechanical Resonators at Low Temperatures
Dissipation of micro- and nano-scale mechanical structures is dominated by
quantum-mechanical tunneling of two-level defects intrinsically present in the
system. We find that at high frequencies--usually, for smaller, micron-scale
structures--a novel mechanism of phonon pumping of two-level defects gives rise
to weakly temperature-dependent internal friction, , concomitant to the
effects observed in recent experiments. Due to their size, comparable to or
shorter than the emitted phonon wavelength, these structures suffer from
superradiance-enhanced dissipation by the collective relaxation of a large
number of two-level defects contained within the wavelength.Comment: To apear in Phys. Rev. Let
Spin-1/2 Heisenberg-Antiferromagnet on the Kagome Lattice: High Temperature Expansion and Exact Diagonalisation Studies
For the spin- Heisenberg antiferromagnet on the Kagom\'e lattice
we calculate the high temperature series for the specific heat and the
structure factor. A comparison of the series with exact diagonalisation studies
shows that the specific heat has further structure at lower temperature in
addition to a high temperature peak at . At the
structure factor agrees quite well with results for the ground state of a
finite cluster with 36 sites. At this temperature the structure factor is less
than two times its value and depends only weakly on the wavevector
, indicating the absence of magnetic order and a correlation length of
less than one lattice spacing. The uniform susceptibility has a maximum at
and vanishes exponentially for lower temperatures.Comment: 15 pages + 5 figures, revtex, 26.04.9
Landau levels in the case of two degenerate coupled bands: kagome lattice tight-binding spectrum
The spectrum of charged particles hopping on a kagome lattice in a uniform
transverse magnetic field shows an unusual set of Landau levels at low field.
They are unusual in two respects: the lowest Landau levels are paramagnetic so
their energies decrease linearly with increasing field magnitude, and the
spacings between the levels are not equal. These features are shown to follow
from the degeneracy of the energy bands in zero magnetic field. We give a
general discussion of Landau levels in the case of two degenerate bands, and
show how the kagome lattice tight-binding model includes one special case of
this more general problem. We also discuss the consequences of this for the
behavior of the critical temperature of a kagome grid superconducting wire
network, which is the experimental system that originally motivated this work.Comment: 18 pages, 8 figure
Quantum Interference on the Kagom\'e Lattice
We study quantum interference effects due to electron motion on the Kagom\'e
lattice in a perpendicular magnetic field. These effects arise from the
interference between phase factors associated with different electron
closed-paths. From these we compute, analytically and numerically, the
superconducting-normal phase boundary for Kagom\'e superconducting wire
networks and Josephson junction arrays. We use an analytical approach to
analyze the relationship between the interference and the complex structure
present in the phase boundary, including the origin of the overall and fine
structure. Our results are obtained by exactly summing over one thousand
billion billions () closed paths, each one weighted by its
corresponding phase factor representing the net flux enclosed by each path. We
expect our computed mean-field phase diagrams to compare well with several
proposed experiments.Comment: 9 pages, Revtex, 3 figures upon reques
Quantum Noise Limits for Nonlinear, Phase-Invariant Amplifiers
Any quantum device that amplifies coherent states of a field while preserving
their phase generates noise. A nonlinear, phase-invariant amplifier may
generate less noise, over a range of input field strengths, than any linear
amplifier with the same amplification. We present explicit examples of such
nonlinear amplifiers, and derive lower bounds on the noise generated by a
nonlinear, phase-invariant quantum amplifier.Comment: RevTeX, 6 pages + 4 figures (included in file; hard copy sent on
request
Imaginary Squashing Mode Spectroscopy of Helium Three B
We have made precision measurements of the frequency of a collective mode of
the superfluid 3He-B order parameter, the J=2- imaginary squashing mode.
Measurements were performed at multiple pressures using interference of
transverse sound in an acoustic cavity. Transverse waves propagate in the
vicinity of this order parameter mode owing to off-resonant coupling. At the
crossing of the sound mode and the order parameter mode, the sound wave is
strongly attenuated. We use both velocity and attenuation measurements to
determine precise values of the mode frequency with a resolution between 0.1%
and 0.25%.Comment: 6 pages, 4 figures, submitted to proceedings of Quantum Fluids and
Solids (QFS) Conference 2006; revised 9/26/0
Elastic Wave Transmission at an Abrupt Junction in a Thin Plate, with Application to Heat Transport and Vibrations in Mesoscopic Systems
The transmission coefficient for vibrational waves crossing an abrupt
junction between two thin elastic plates of different widths is calculated.
These calculations are relevant to ballistic phonon thermal transport at low
temperatures in mesoscopic systems and the Q for vibrations in mesoscopic
oscillators. Complete results are calculated in a simple scalar model of the
elastic waves, and results for long wavelength modes are calculated using the
full elasticity theory calculation. We suggest that thin plate elasticty theory
provide a useful and tractable approximation to the full three dimensional
geometry.Comment: 35 pages, including 12 figure
Hidden spin-current conservation in 2d Fermi liquids
We report the existence of regimes of the two dimensional Fermi liquid that
show unusual conservation of the spin current and may be tuned by varying some
parameter like the density of fermions. We show that for reasonable models of
the effective interaction the spin current may be conserved in general in 2d,
not only for a particular regime. Low temperature spin waves propagate
distinctively in these regimes and entirely new ``spin-acoustic'' modes are
predicted for scattering-dominated temperature ranges. These new
high-temperature propagating spin waves provide a clear signature for the
experimental search of such regimes.Comment: 4 pages, no figures, revised version, accepted for pub. in the PR
Magnetization plateau in a two-dimensional multiple-spin exchange model
We study a multiple-spin exchange model on a triangular lattice, which is a
possible model for low-density solid 3He films. Due to strong competitions
between ferromagnetic three-spin exchange and antiferromagnetic four-spin one,
the ground states are highly degenerate in the classical limit. At least
2^{L/2}-fold degeneracy exists on the L*L triangular lattice except for the
SO(3) symmetry. In the magnetization process, we found a plateau at
m/m_{sat}=1/2, in which the ground state is "uuud state" (a collinear state
with four sublattices). The 1/2-plateau appears due to the strong four-spin
exchange interaction. This plateau survives against both quantum and thermal
fluctuations. Under a magnetic field which realizes the "uuud" ordered state, a
phase transition occurs at a finite temperature. We predict that low-density
solid 3He thin films may show the 1/2-plateau in the magnetization process.
Experimental observation of the plateau will verify strength of the four-spin
exchange. It is also discussed that this magnetization plateau can be
understood as an insulating-conducting transition in a particle picture.Comment: 10 pages, RevTeX, 12 figures, added a reference and corrected typos,
to be published in Phys.Rev.B (01 APR 99
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